The invention relates to a trip device with phase reconstitution comprising:
at least two sensors arranged on main conductors to be protected, and
a processing unit comprising means for processing the current signals connected to the sensors, means for phase reconstitution receiving signals representative of the current signals supplied by the sensors, and means for processing tripping functions connected to the means for processing the current signals and to the means for reconstitution. The invention also relates to a circuit breaker comprising such a trip device.
Trip devices with phase reconstitution generally comprise two sensors arranged on two main phase conductors and reconstitute a signal representative of the third phase when the power system is a three-phase system. Each sensor can also surround two main conductors so that measurement of the current will be representative of the sum of two phase currents. These types of trip devices are described in particular in the U.S. Pat. Nos. 5,777,835 and 5,815,357.
For reconstitution of a phase which is not measured, sampling means supply current signal samples at regular intervals. Processing means comprising a microprocessor then compute the missing phase signal point by point.
In this processing mode, computation of a missing phase is liable not to be sufficiently precise. Current sensors, such as magnetic circuit current transformers, do in fact comprise non-linear responses. The non-linearity of the responses is due in particular to saturation of the magnetic circuits at high currents.
Thus, as soon as the currents in the main conductors increase, point by point reconstitution becomes imprecise.
The object of the invention is to achieve an electronic trip device with phase reconstitution enabling precision of reconstitution to be achieved over a wide measuring range.
In a trip device according to the invention, the means for phase reconstitution comprise:
first determining means to determine signals representative of the amplitudes of the current signals supplied by the two sensors,
second determining means to determine a signal representative of a phase shift between signals representative of the signals supplied by the two sensors, and
third determining means connected to the first and second determining means to determine a signal representative of a phase current reconstituted according to the signals representative of the amplitudes and to the signal representative of a phase shift of the signals supplied by the two sensors.
In a preferred embodiment, the second determining means comprise means for computing the cosine of the phase shift between the signals representative of the signals supplied by the two sensors.
The second determining means preferably comprise means for determining times when the signals supplied by the current sensors pass zero in a predetermined direction, said times being a first time when a first signal passes zero, a second time when a second signal passes zero and a third time when a first signal again passes zero, a signal representative of the phase shift being appreciably proportional to the ratio of a time between the first and second times over a time between the first and third times corresponding to the period of the first signal.
In a preferred embodiment, the third determining means comprise first means for computing a product signal to compute the product of the signals representative of the amplitudes and of the signal representative of a phase shift.
Advantageously, the third determining means comprise second means for computing a sum signal to compute the sum of the squared amplitude signals and of the product signal.
The trip device preferably comprises means for determining zero-phase sequence current comprising a current sensor arranged on all of the main conductors.
In a preferred embodiment, the means for reconstitution use the signal supplied by the third determining means when at least one of the current signals supplied by the two sensors exceeds a first preset threshold.
Advantageously, each current sensor surrounds two main conductors.
In a four-pole embodiment, the trip device comprises at least three sensors arranged on main conductors to be protected, the first determining means determining signals representative of the amplitudes of the current signals supplied by the sensors, and the third determining means connected to the first and second determining means determining a signal representative of a phase current reconstituted according to the signals representative of the amplitudes, to the signal representative of a phase shift of the signals supplied by two sensors, and to a signal supplied by a third sensor.
A circuit breaker according to an embodiment of the invention comprises at least three contacts connected in series with main conductors and a trip device having the features defined above.